In public sewer systems sanitary sewage and rain water are usually collected in a single system of pipes called a combining system. The dry-weather flow collected in such a system is mechanically and biologically purified in a sewage purification plant. According to usual regulations the collecting line which leads to the purification plant must be capable of holding at least five times the amount of dry-weather flow or a critical quantity of rain water. The combined water, which in the case of heavy rain exceeds the volumetric capacity of the main sewer, passes via a rain outlet or overflow gate, arranged so as to follow the draining area, directly into the receiving water (free body of water). The pollution of bodies of water resulting therefrom which has to be avoided for hygienic reasons and considerations related industrial water development is intensified by the fact that at each rainfall the first tide wave is strongly polluted and furthermore by the rinsing effect exerted in the sewer system such tide wave is strongly enriched with sanitary sewage particles.
A known device consists in replacing the rain outlet by a rain-storing tank which stores the first, strongly polluted, combined water and feeds it at a time lag to the purification plant and/or purifies it mechanically by the detention period before discharging it into the receiving water. This permits use of a small diameter of the combined water line leading to the purification plant, which diameter can thus be adapted to the efficiency of the biological section of the purification plant. Notwithstanding such advantages, rain-storing tanks are rarely used. In known construction types only a small portion of the polluted sediment is conveyed, after the end of the rainstorm, into the purification plant because the discharged water produces no sufficient sweeping force for carrying off the sediments. Therefore a frequent cleansing by shoveling or spraying becomes necessary.